Propylene-based polymers are generally widely utilized for automotive interior trim or automotive exterior trim such as fender, bumper, side molding, mud guard and mirror cover by subjecting the polymers to injection molding, because of their excellent rigidity, hardness and heat resistance.
Moreover, a polypropylene composition having been improved in impact resistance by adding polyethylene, a rubber component, a non-crystalline or low-crystalline ethylene/propylene copolymer (EPR), a non-crystalline ethylene/α-olefin copolymer or the like to a propylene-based polymer or by directly polymerizing them according to the use purpose and a polypropylene composition to which an inorganic filler such as talc has been further added in order to make up for rigidity lowered by blending the rubber component are also well known. In the case of such propylene-based polymers, however, further weight-lightening and wall-thinning of their molded articles have been desired, and in order to obtain molded articles not only realizing them but also having sufficient strength, a propylene-based polymer having more improved balance between rigidity and impact resistance (that is, excellent in both of rigidity and impact resistance) or a composition comprising the polymer has been desired. Furthermore, a propylene-based polymer capable of improving a molded product appearance defect called “flow mark” caused by irregular flow of a molten resin formed in the molding process or a composition comprising the polymer has been also desired.
As a means to solve such a problem, a technique of widening a molecular weight distribution of a propylene-based polymer is generally known. In particular, a propylene-based polymer containing a high-molecular weight component exerts an excellent effect. In the past, there have been made a large number of reports that the balance between rigidity and impact resistance has been improved by, for example, a method wherein a polymer having different molecular weights is prepared by multi-stage polymerization to widen a molecular weight distribution of the polymer (Japanese Patent Laid-Open Publication No. 170843/1993 (patent literature 1)), a method wherein a catalyst containing plural kinds of electron donors is used (Japanese Patent Laid-Open Publication No. 7703/1991 (patent literature 2)), and a method wherein an olefin polymerization catalyst using a solid titanium catalyst component and using an aluminum alkyl compound and a nitrogen-containing aliphatic silicon compound as co-catalysts (Japanese Patent Laid-Open Publication No. 120021/1996 (patent literature 3)) is used to widen a molecular weight distribution. Moreover, a method wherein various propylene resin compositions are mixed to improve appearance properties with maintaining rigidity and impact resistance (Japanese Patent Laid-Open Publication No. 163120/2008 (patent literature 4)), etc. have been also reported. Furthermore, there have been made a large number of reports that a titanium trichloride catalyst (TiCl3) hitherto well known is also a catalyst capable of achieving widening of a molecular weight distribution (Japanese Patent Laid-Open Publication No. 34478/1972 (patent literature 5)).
In the patent literatures 1 and 2, however, the molded product appearance has not been improved though the balance between rigidity and impact resistance has been improved. In the patent literature 3, improvement in flow mark of molded product appearance is insufficient. In the patent literature 4, flow mark of molded product appearance is improved, but the propylene/ethylene rubber component of a high molecular weight is aggregated to sometimes cause a defect of graininess on the molded product surface. In the patent literature 5, widening of a molecular weight distribution can be achieved by taking advantage of features of the catalyst, but because of a large amount of a residual metal due to low activity, of the catalyst, deashing step is necessary, or because of extremely low stereoregularity, there is a disadvantage that rigidity cannot be increased.
The present invention has been made in view of such prior art as mentioned above, and it is an object of the present invention to obtain, with a high activity, a propylene-based polymer which is excellent in both of rigidity and impact resistance and is very excellent also in its molded product appearance and to provide a composition containing the polymer and molded products obtained therefrom.